Relationship between protein kinase C and adenylyl cyclase activity in the regulation of growth hormone secretion by human pituitary somatotrophinomas

Ghrelin reduces voltage-gated calcium currents in GH₃ cells via cyclic GMP pathways

Ghrelin is an endogenous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca2+ concentration ([Ca2+]i), which is determined by Ca2+ influx and release from intracellular Ca2+ storage sites. Ca2+ influx is via voltage-gated Ca2+ channels, which are activated by cell depolarization. The mechanism underlying the effect of ghrelin on voltage-gated Ca2+ channels is still not clear. In this report, using whole cell patch-clamp recordings, we assessed the acute action of ghrelin on voltage-activated Ca2+ currents in GH3 rat somatotrope cell line. Ca2+ currents were divided into three types (T, N, and L) through two different holding potentials (-80 and -40 mV) and specific L-type channel blocker (nifedipine, NFD). We demonstrated that ghrelin significantly and reversibly decreases all three types of Ca2+ currents in GH3 cells through GHS receptors on the cell membrane and down-stream signaling systems. With different signal pathway inhibitors, we observed that ghrelin-induced reduction in voltage-gated Ca2+ currents in GH3 cells was mediated by a protein kinase G-dependent pathways. As ghrelin also stimulates Ca2+ release and prolongs the membrane depolarization, this reduction in voltage-gated Ca2+ currents may not be translated into a reduction in [Ca2+]i, or a decrease in GH secretion.

GHRP-6 induces CREB phosphorylation and growth hormone secretion via a protein kinase Csigma-dependent pathway in GH3 cells

This study examined the effect of GHRP-6, a known GHSs receptor agonist, on the phosphorylation of cAMP-responsive element-binding protein (CREB) and the underly mechanism. GH3 cells were cultured and subjected to different treatments as follows: GHRP-6, GHRP-6 plus GHRH, phorbol ester (PMA), an activator of PKC, alone or in combination with GHRP-6, Gö6983, a general inhibitor of PKCs, in the presence or absence of GHRP-6, rottlerin, an inhibitor of PKCs, alone or plus GHRP-6. The cells were transiently transfected with PKCsigma-specific siRNA and then treated with GHRP-6. GH level was measured by enzyme-linked immunosorbent assay (ELISA). The expression of phosphor-CREB, PKCsigma, PKCtheta and phosphor-PKCsigma was determined by Western blotting. The results showed that GHRP-6 stimulated GH secretion in both time- and dose-dependent manners and enhanced the effect of GHRH on GH secretion. GHRP-6 was also found to induce CREB phosphorylation. Moreover, GH secretion was enhanced by the PKC activator PMA and reduced by the PKC inhibitors (Gö6983, rottlerin) and knockdown of PKCsigma. PKCsigma could be activated by GHRP-6. It is concluded that PKC, especially PKCsigma, mediates CREB phosphorylation and GHRP-6-induced GH secretion.

The in vitro regulation of growth hormone secretion by orexins

Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.

Preliminary study on the relationship between cAMP level and gsp expression in cultured human pituitary somatotrophinomas

In order to investigate the relationship between abnormal intracellular signal transduction and tumorgenesis of human pituitary somatotrophinomas, the effects of protein kinase A (PKA)-dependent growth hormone (GH) releasing hormone (GHRH) and protein kinase C (PKC)-dependent GH-releasing peptide (GHRP-6) on cAMP production were observed by using cell culture and biochemical methods, and the expression of the gsp oncogene was detected by using PCR and direct sequence assay methods in 11 patients with human pituitary somatotrophinomas. It was found that GHRP-6 exerted significant stimulatory effect on cAMP production by 2 gsp-positive tumors and no effect on the gsp-negative tumors. GHRP-6 could enhance the stimulation of cAMP production induced by GHRH in tumor without gsp oncogenes. It was suggested that both GHRH and GHRP-6 exert identical effects on human pituitary soamtotrophinomas, which was contributed to the cross-talk between the two intracellular signal transduction pathways in pituitary cells.

The role of protein kinase C and its effect on GHRH in the regulation of hormone secretion by somatotrophinomas

Phorbol ester-induced release of growth hormone (GH) and prolactin (PRL) from human somatotrophic tumors was examined in vitro. 12-O-tetradecanoyl-phorbol-13-acetate (TPA) strongly stimulated GH and PRL secretion and showed an additive effect on GH secretion if used in combination with GH releasing hormone (GHRH). In contrast, staurosporine exerted a variable inhibitory effect on GH release. There was no correlation between such effects and gsp mutations. The findings suggested that TPA doesn't act directly through cAMP signal transduction system.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

The effect of GHRH, GHRP-2 and somatostatin on GH secretion by fetal pituitary

Growth hormone releasing peptide (GHRP-2) is a synthetic hexapeptide which specifically stimulates secretion of growth hormone (GH) by fetal pituitary somatotrophs through a new membrane receptor, which is different from growth hormone releasing hormone (GHRH) and somatostatin (SMS) receptors. We used cell cultures of human fetal pituitary somatotroph cells to investigate the effect of GHRH, GHRP-2 and somatostatin on GH secretion. The results showed that the mechanism of GHRH/SMS and GHRP-2 was different. This indicated that a different intracellular signal transduction system might also play a crucial role in the regulation of GH secretion.

Hormone secretion by cell culture of human GH-PRL secreting pituitary adenomas: effects of bromocriptine

Dopamine agonists effectively reduce the secretion of prolactin (PRL) in the great majority of prolactinomas and reduce the bulk of the adenomas, as well as have partial therapeutic effect on some patients with acromegaly. The inhibitory effect of bromocriptine (BC), a dopamine agonist, on growth hormone (GH) and PRL secretion of dispersed cells from the pituitary adenomas of 16 cases of acromegaly, which secret GH and PRL simultaneously, were evaluated in vitro. The significant inhibitory effects of BC on PRL secretion were found in 12 cases. It was also found that PRL secretion was strongly inhibited when GH was suppressed; on the contrary, when GH secretion was not suppressed, the production of PRL was not or weakly inhibited. The exact mechanism of the effects is unclear so far. It is necessary to investigate, at molecular level, the etiology of GH-PRL adenomas and its response to therapeutic agents.

Relationship between GHRP-6 and TPA in the regulation of growth hormone secretion by human pituitary somatotrophinomas

Growth hormone releasing peptide (GHRP-6) is a synthetic hexapeptide which specifically stimulates secretion of growth hormone (GH) by pituitary somatotrophs. Phorbel ester, 1, 2 tetradecanoylphorbol 13 acetate (TPA) can also stimulate releasing of GH. The precise intracellular mechanism has not been entirely deciphered. We used cell cultures of human pituitary somatotrophinomas to investigate the relation between GHRP-6 and TPA on membrane phosphatidylinositol (PI) turnover and GH secretion. The results showed that the working mechanisms of GHRP-6 and TPA are not identical, although they all can stimulate GH secretion in human pituitary somatotrophinomas. This indicates that PI-PKC signal transduction system may play a crucial role in the regulation of GH secretion.